Negative conductance amplifier



NOV. 26, 1963 R, STEINHOFF 3,112,454

NEGATIVE CONDUCTANCE AMPLIFIER Filed NOV. 23, 1959 3 Sheets-Sheet 1 Fig. 1. PLQ-Z- /Z/ La Z4 Z0 VP-fh:

J4 jo YW .fz

PLQ. 4

Nov. 26, 1963 R. sTErNHoFF r3,112,454

NEGATIVE CONDUCTANCE AMPLIFIER Filed Nov. 2a, 1959 5 sheets-'sheet 2 @l l L Il I Y ,Il fn Fig".

INVENTOR. Reqnold Steinhoff Afton-gea Nov. 26, 1963 R. sTr-:INHOFF 3,112,454

NEGATIVE CONDUCTANCE AMPLIFIER Filed Nov. 23, 1959 3 Sheets-Sheet 3 V5 *l Fig. 16.

Fig. l5.

INVENTOR Reqnold Steinhoff l At torney r"United States Patent fiice waist Patented Nov. 26, 1963 3,112,454 NEGATIVE CONDUCTAN CE AMPLIFIER Reynold Steinhoii, Livingston, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed Nov. 23, 1959, Ser. No. 854,953 9 Claims. (Cl. S30-61) This invention relates to high frequency signal translating systems, and more particularly to high frequency amplifying circuits including negative resistance devices.

In an amplifier circuit including a negative resistance device as the active circuit element thereof, the total positive conductance of the signal source and load circuits must be selected so that the combination thereof with the negative conductance of the device produces a resultant positive conductance. If a resultant negative conductance is produced, the circuit tends to oscillate. In general, the signal source circuit conductance, the load circuit conductance and the negative resistance device conductance are functions of frequency. Thus, although the various conductances satisfy the conditions for amplification over a desired frequency band, at some frequencies outside of the desired frequency band the combination of these conductances may produce a resultant negative conductance which causes instability as evidenced by spurious or parasitic oscillation. Such instability detrimentally affects the operation of an amplifier circuit.

It is accordingly an object of this invention to provide an improved high frequency signal translating system of the type including a negative resistance device as the active element thereof.

It is another object of this invention to provide an improved high frequency amplifier of the type using a negative resistance device which is not subject to spurious oscillation outside of the amplifier frequency passband.

A high frequency amplifier in accordance with the invention includes a negative resistance device, such as a negative resistance diode, coupled to suitable signal source and load circuits. The negative resistance device is also coupled to frequency responsive means which may include a resonant transmission line structure or its equivalent. Where the negative resistance device is of the voltage controlled type, the frequency responsive means is connected in parallel with the device and is selected: (l) to present a positive conductance that exceeds the absolute value of the negative conductance of the device for frequencies outside of the desired amplifier passband; and (2) is less than the negative conductance of the device for frequencies falling Within the amplifier passband. The combination of the negative resistance device and the frequency responsive means provides a resultant positive conductance for frequencies outside of the amplifier passband, and, therefore, parasitic oscillations do not occur. Within the amplifier passband this combination exhibits a resultant negative conductance to the signal input and output circuits so that amplification can be obtained. Where current controlled negative resistance devices are used, the circuit is the dual of the circuit including the voltage controlled device. In such a case, the positive resistance of the frequency responsive means connected in series with the device is selected to exceed the negative resistance of the device for frequencies outside of the amplifier passband and to be less than the negative resistance of the device for signal frequencies within the amplifier passband.

A frequency responsive means for producing the aforementioned characteristic may comprise a properly loaded electrical resonator means, such as, for example, a transmission line or its equivalent, which is resonant at the center frequency of the amplifier passband. With a negative resistance device connected to such a resonator means, the susceptance of the device and the susceptance of the resonator means produce phase shifts in the applied signal which may result in reflected waves which are of a phase to interfere with or cancel the signal voltage across the negative resistance device, thereby preventing efficient amplification. To prevent improper phase shifts of this type, second resonator means is coupled to the first resonator means at a position adjacent the negative resistance device connection. The second resonator means is selected to resonate with the combined susceptance of the negative resistance device and first resonator means at the desired frequency of amplification.

The novel features that are considered to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIGURE 1 is a graph illustrating the current-voltage characteristic of a voltage controlled negative resistance diode;

FIGURE 2 is a schematic circuit diagram of a D.C. biasing circuit for the diode of FIGURE 1;

FIGURE 3 is an equivalent circuit diagram of a two terminal negative resistance signal translating stage;

FIGURE 4 is a diagrammatic representation of a resonant transmission line structure for use with high frequency amplifying circuits embodying the invention;

FIGURE 5 is a graph illustrating the input conductance characteristic of the transmission line structure shown in FIGURE 4;

FIGURE 6 is a plan view of a resonant transmission line structure used in high frequency amplifier circuits embodying the invention;

FIGURE 7 is a sectional View of the transmission line structure shown in FIGURE 6 taken on section lines 7 7;

FIGURE 8 is a diagrammatic representation of another form of transmission line structure embodying the invention;

FIGURE 9 is a graph showing the input conductance of the transmission line structure shown in FIGURE 8;

FIGURE 10 is a graph illustrating the input conductance of the transmission line structure shown in FIGURE 4 when the transmission line characteristic impedance is mismatched to its load;

FIGURE l1 is a plan view of a resonant transmission line structure for high frequency amplifier circuits embodying the invention;

FIGURE l2 is a sectional view of the transmission line structure shown in FIGURE 11 taken on section lines 11-11;

FIGURE 13 shows diagrammatically a modification of the structure shown in FIGURE 4 for use with current controlled negative resistance diodes;

FIGURE 14 is a diagrammatic sectional view of a coaxial transmission line amplifying system in accordance with the invention;

FIGURE 15 shows diagrammatically a modification of the structure shown in FIGURE 8 for use with current controlled negative resistance diodes; and

FIGURE 16 is a diagrammatic sectional view of a coaxial transmission line amplifying system embodying the invention.

The principles of the present invention are particularly, although not exclusively, applicable to signal translating systems using negative resistance diodes. Negative resistance diodes may be divided into two general categories: current controlled negative resistance diodes, an example of which is described in Shockley U.S. Patent 2,855,524; and voltage controlled negative resistance diodes, an example of which is described by H. S. Sommers, Proceedings of the IRE, July 1959, pages 1201-1205.

The current-voltage characteristic of a typical voltage controlled negative resistance diode suitable for use in amplifying circuits embodying the invention is show-n in FIGURE 1. The current scales depend on the area and doping of the diode junction, but representative currents are 4in the milliampere range. For a small voltage in the back direction, the back current of the diode increases as the reverse bias voltage is increased as indicated by the region b of FIGURE 1. This increased back current is thought to be due to an increase in quantum mechanical tunneling current.

For small forward bias voltages, the characteristic is substantially symmetrical with region b (FIGURE 1, region c). The forward current is believed to result `due to a reduction in the reverse quantum mechanical tunneling current. At higher forward bias voltages, about 50 millivolts (mv.), the forward current reaches a maximum (region d, FIGURE 1), and then begins to decrease. This drop continues (FIGURE 1, region e) until eventually at about A35()` mv., normal injection over the barrier becomes important and the characteristic turns into the usual forward behavior (region f, FIGURE 1).

The negative resistance of the diode is the incremental change in voltage divided by the incremental change in current, or the reciprocal slope of the region e of FIG- URE 1. To bias the diode for stable operation in the negative resistance region of its characteristic requires a suitable voltage source having a smaller internal irnpedance than the negative resistance of the diode. As shown lin FIGURE l2, the voltage source 18 may comprise a battery 22 and a variable resistor 24, with the internal resistance of the source 1S being the sum of the internal resistance of the battery 22 and the adjusted resistance of the variable resistor 24, Such a voltage source has a D.C. load line 26 as indicated in FIGURE 1, which is characterized by a current-voltage relationship which has a greater slope than the negative slope of the diode characteristic and intersects the diode characteristic at only a single point. If the voltage source 118 has an internal resistance which is greater than the negative resistance of the diode, the source would have a load line 2S w-ith a smaller slope than the negative slope of the diode characteristic as indicated in FIGURE 1, and would intersect the diode characteristic curve at three points. Under the latter conditions the diode is not stably biased in the negative resistance region. This lack of stability is because an incremental change in current through the diode due to transient or noise currents, or the like, produces a regenerative reaction which causes the diode to assume one of its two stable states represented by the intersection of the load line 2S with the positive resistance portions of the diode characteristic curve.

`A simplified alternating current (A.C.) circuit diagram 'of a two terminal negative resistance amplifier circuit is shown in FIGURE 3. The negative resistance diode is represented as an equivalent conductance Gd and is stably biased in the negative resistance region of its characteristic, preferably at the minimum negative resistance point, by a D.C. biasing circuit, not shown. The equivalent conductance of the source and load circuits connected to the diode are indicated as Gg and G1 respectively.

The power gain for this circuit is equal to:

Power out Power in where Power is the maximum power that may be delivered to a load from a given generator.

Assuming optimum impedance matching between the source Gg and the load G1, the maximum power that may be delivered from the source Gg to the load G1 may be expressed:

(1) Power ira-:E261 Wherein E is the source voltage but I 2 E GNLG.l

Wherein I is the total current and G1=Gg for optimum matching substituting 2 in Formula 1 The power out may be expressed:

(4) Power out=E2G1 but I (5) E Gciwcd substituting 5 in Formula, 4

L (6) Power out-(CTYE GII G)2 Thus Power Guasipati@ :Laici-gpg OVVQT 1D L2- (7) Fewer Gam (Gg+G1- Gd)2 Thus for high gain, Gd, the negative conductance of the diode, should approach, but be 'less than, the total positive conductance of the circuits, Gg-i-Gl.

In general it may be said that the conductances of the signal source circuit, the load circuit, and the negative resistance diode, are :functions of frequency. Thus the circuit may be stable at some frequencies and unstable at others. For example, although the amplifier has been designed so that the circuit is stable over a predetermined desired frequency passband, the negative conductance of' the diode may exceed the positive conductance of source: and load circuits at frequencies outside the passband of the amplifier. Under these conditions parasitic or spurious oscillation will occur. This seriously degrades'. the amplifier performance, and results in distortion and loss of gain.

In accordance with the invention, stable gain may bev achieved over a predetermined desired frequency pass-- band by introducing a frequency dependent conductance: device into the negative resistance diode circuit. The:

frequency dependent conductance device is selected soy 4. In this gure, a resonant transmission line structure includes a pair of parallel conductors 32 and 30. A noninductive resistor 34 of iow resistance, such as a slab of germanium or graphite Gr, is connected between the conductors 32 and 3G near the end thereof provided with a pair of input terminals 36. The length l1 of conduc- Itors 32 and 30 between the resistor 34 and the open cirouited ends of the line is selected to be a quarter wave length at the desired center frequency of the amplifier passband, and the length l2 between the resistor 34 and the terminals 36 is very much smaller than the length l1. The input admittance Yin of the transmission line structure looking into the terminals 32, has a real component Gm and imaginary component jBm.

If the conductance of the resistor 34 is made larger than t-he absolute value of the negative conductance of a diode to be connected between the terminals 36, then the structure shown in lFIGURE 4 will satisfy the conditions that the positive conductance thereof will exceed that of the negative conductance of the diode for frequencies outside of the amplifier passband and will exhibit a substantially zero conduct-ance for signal frequencies within the amplifier passband. The real component, i.e., conductance, of the transmission line admittance plotted against frequency is shown in the graph of FIGURE 5. At zero frequency, the input conductance of the transmission line structure of FIGURE 4 is equal .to the conductance of the resistor 34 or Gr, lwhich in the present case is selected to match the characteristic impedance Go, of the quarter wave section l1. The conductance Gr is greater than the absolute value of the negative conductance of the diode to be used as indicated in FIGURE 5 by the dashed line labeled -Gd. For input signal frequencies corresponding to the frequency of resonance fol of the quarter wave transmission line section l1, a voltage minimum occurs across the terminals of the resistor 34. Since there is a voltage minimum, minimum current iiows through the resistor 34 and the effective conductance at the terminals 36 is minimum. This minimum conductance is less than the absolute value of the diode negative conductance Gd. Since the resistor 34matches the impedance G0 of the transmission line structure =(a1l of conductors 32 and 30), the length l2 of the conductors 32 and 3) has substantially no effect on the input conductance of the Itransmission line. As the signal input frequency is increased above the frequency fol, the input conductance of the transmission line again increases to the maximum with a sharp minimum occurring at every odd harmonic of the frequency fm. By selecting a negative resistance diode which has a cut-off frequency below the third harmonic of fol it was found that no diiculty was experienced with parasitic oscillations.

A physical exempliiication of a transmission line structure of the type described is shown in FIGURES 6 and 7. This transmission -line structure may comprise symmetrioal parallel conductors or a single conductor spaced from a ground plane. The latter configuration is shown in FIGURES 6 and 7. This transmission line structure, which was used in a negative resistance amplilier operating in the range of 400 megacycles/second, comprises three elements. The irst of these elements is a conductor section 4t) having a length l1, which corresponds to the length I1, shown in FIGURE 4. The second element is a conductor section 42 having a length l2 which corresponds -to the length l2 shown in FIGURE 4. The third element is a tuning stub 44 of conductive material which is provided for tuning out the susceptances presented by the negative resistance diode and the conductor section 42 as will be described hereinafter. The transmission line structure is connected by an impedance transformer comprising a series of conductive sections 46, 4S, 50 and 52 of decreasing width and a coaxial transmission line 54, to suitable amplifier input and output circuits, represented by the conductances Gs and G1. The sections 6 46-52 are stepped in width to provide a smooth impedance transformation over a -wide band of frequencies.

The transmission line structure and impedance transformer may comprise a single continuous sheet of conductive material which is insulated and spaced from a ground plane member 56 which comprises a rectangular sheet of conductive material. A negative resistance diode 58 is connected to the transmission line structure between the ground plane member 56 and the end of the conductor section 42 adjacent the impedance transformer. A non inductive resistor i60 is connected between fthe ground plane member `56 and the junction of the conductor sections 40' and 42 thereof.

The biasing voltage for the negative resistance diode 58 is supplied from a regulated power supply, not shown, connected with the terminals 62. By \way of example the power supply provides a regulated biasing voltage for the diode which is on the order of mv. The biasing voltage is developed lacross the resistor 60, and the combined parallel resistance of the resistor 60 and internal resistance of the power supply is less than the absolute value of the negative resistance of the diode so that the diode will be stably biased. If desired, suitable isolating means such as a radio frequency choke coil (indicated) may be provided in series with the power supply connections to keep the low power supply resistance from load ing the amplifier. To provide further isolation, the power supply is connected to the transmission line structure adjacent the noninductive resistor 60 which is at a rvoltage minimum at amplifier `operating frequencies in the passband. This has the advantage of suppression of parasitic oscillation #which might occur as is described in the copending application of H. S. Sommers entitled Negative Resistance Diode Circuit tiled July 7, 1959, Serial No. 825,483.

As shown -in FIGURE 5, the transmission line structure presents a greater positive conductance than the absolute value of the conductance of the diode for all frequencies except where the transmission line conductance curve drops to a minimum at the lresonant frequency of the transmission line and odd harmonics thereof. This means that the combination of the diode and the transmission li-ne structure does not provide a resultan-t or net negative conductance except near the resonant `frequency of the transmission line -section l1 or odd harmonics thereof. The circuit is so designed that the signal source and load conductances exceed the negative conductance of the diode by a small amount to provide amplification at the frequency fol. The transmission line characteristic determines the amplifier frequency passband which is indicated by 26 of 'FIGURE 5. As a practical matter the diode is selected so that it cannot oscillate at frequencies above the second harmonic so that parasitic oscillation cannot take place at third or higher odd harmonics of the frequency fol.

The diode 58 has capacitance across its junction which is presented between the conductors of the transmission line structure. Furthermore the `'conductor section 42, length l2, has reactance which is present in the circuit. Although the conduct-or section 401 of the transmission line is matched to its load G1, the reactive or susceptance elements of the diode 58 and conductor section 42 may cause phase shifts which adversely affect the amplifier operation, by causing the signal across the diode 53 to be phase shifted i-n such ia manner that :the signal is reilected from the open end of the transmission line back to the diode in a phase which tends to cancel the signal voltages thereacross. If the negative susceptance (capacitance) presented by the diode 58- is equal to the positive susceptance (inductance) of the conductor section 412 Ithen these sus'ceptances balance out and the reiiections would be inthe proper phase to obtain efficient amplification. However, as a practical matter, it is very difficult to adjust or design these elements to provide equal and opposite susceptance values and therefore means are provided for prevent-ing undesirable signal Iwave reiiection due to the phase shifts produced by these susceptances from ad-versely affecting the amplifier operation.

Further in accordance with the invention, a tuning stub 44, of length I3 is connected to the conductor sections 40 and 42 at point adjacent the diode 58. The length I3 of the tuning stub 44 is selected to resonate at the amplifier frequency with the susceptances in the circuit introduced by the diode 53 and the conductor section 42. The effective length of the tuning stub 44 may be varied by changing the position of a rotatable conductive tuning member 45 which is in variable contacting registry with the tuning stub 44. The tuning member 45 is supported by an insulating suppont 47 which has `an arm 49 that is pivotally mounted on the insulating support of the transmission line structure. When the incidental susceptances are tuned out, the reliections of the signal voltage down the transmission line 4are in phase with the input signal voltage applied to the diode 58 thereby enabling efficient amplifier operation.

lIf the transmission line struc-ture is mis-matched to its load7 vsuch as when the conductance value of the resistor 34 is larger than the characteristic admittance G0 of the transmission line, lthen the resultant input conductance characteristic will appear as shown in FIGURE l0. As shown, the input conductance decreases to a minimum at the odd harmonics fm of the resonant frequency of the quarter wave conductor section 46 represented by the length l2 of FIGURES 4 and 6. However, because of the mismatched condition, the envelope of the transmission lline conductance decreases to a minimum at a frequency fog. The lfrequency fog is the resonant frequency of -the quarter Iwave transmission line conductor section 42. To eliminate the dips in the input conductance which occur at the odd harmonics of the res-onant frequency fo portion of the conductor section 40, the conductor section 40 may be removed as shown in FIGURE 8. The structure of FIGURE 8 has an input conductance characteristic as shown in the graph of FIGURE 9. It will be noted that the frequency bandpass characteristic corresponding to the frequency band 2A, of an amplifier circuit using such a structure has been considerably widened.

A physical exemplification of a structure for producing the input conductance characteristic of FIGURE 9 is shown in FIGURES 1l and l2. In these FIGURES, the length l2 of a conductor section 76y corresponds to the length l2 of the parallel conductors shown in FIGURE 8. A tuning stub 72 having a length I3 is provided for tuning out the yresidual susceptances present in the circuit which are primarily due to the negative resistance diode Sti. if desired, a variable tuning element similar to the element 45 of FIGURE 6 may be added to adjust the effective length of the tuning stub 72,. An impedance matching line section 74 having a length I4 is provided to couple the transmission line conductor section 79l to a co-axial transmission line 76 which in turn is connected to the source and load circuits for the amplifier.

The transmission line structure shown in FIGURES ll and l2 is similar to that shown in FIGURES 6 and 7 in that the :sections 7o, 72, 74 comprise a substantially continuous conductive sheet which is spaced from a conductive grou-nd plane of rectangular configuration 78 by a suitable insulating material. The negative resistance diode 80 and. a non-inductive resistor which may comprise a slab of germanium S2 are connected between the ground plane and the conductor section 70 at opposite ends thereof. The diode 50 lis biased to the desired point in its operating characteristic, so that the diode exhibits a stable negative resistance, by la suitable power supply, not shown, which -is :connected to a pair of terminals 84 -and 86. As mentioned above in connection with FIG- URE. 6, the power supply is connected between the ground plane and the conductor section 79 adjacent the non-inductive resistor 82.

Ehe conductor section 7G is made a quarter-Wave length long for the mid-band frequency of the amplifier. The characteristic admittance of this section is indicated as Yin in FIGURE 8 of the drawings. In order that there exists an amplifying region Ishown as 2A in FIGURE 9, the characteristic impedance of the conductor section 70,

Goi NGdGr The precise values used will in turn define the upper and llower frequency limits of amplification. lFor stability outside of the amplifying region, the further relationship |--Gd| Gr must also be satisfied.

High frequency amplifiers lnay employ current controlled negative resistance devices as shown 'in FIGURES 13 to 16. In general these circuits are the duals of those shown and described heretofore. For example yFIGURE 13 is a diagrammatic representation of a transmission line `structure fusing a current controlled negative resistance diode, which is the dual of the structure of FIGURE 4. As shown in FIGURE 13 the transmission line comprises a pair of parallel conductors 30 and 32' having a pair of section-s of length l1 and l2 respectively. A resistor 34" of small physical length is inserted in series with the conductor 3ft'. The resistance value of the resistor 34 is selected to be greater than the negative resistance of a current controlled negative resistance diode to be connected in series with one of the conductors 3G and 32' adjacent the input terminals 36. When the characteristic impedance of the shorted quarter wave transmission line section represented by the length l1 is matched to its load, then the section represented by the length l2 has little eect on the input resistance for any frequency. The input resistance at the terminals 36 is represen-ted by the graph of FIGURE 5, it being understood that the ordinate is now to be read in terms of resistance rather than conductance as shown. In other words G1. of FIGURE 5 represents the resistance of the resistor 34', j-Gdl represents the absolute value of the negative resistance of the diode, and Gin represents the input resistance between the terminals 36.

From FIGURE 5, it can be seen that the positive resistance of the transmission line structure exceeds the absolute value of the negative resistance of the diode Rd for all frequencies except at the frequency of resonance fol of the shorted quarter wave transmission line section, l1, and third harmonics thereof. Since the resistor 34' is in series with the diode, then the combination of the transmission line structure and the diode can present a negative resistance to the source and load circuits only at frequencies in the range of frequency fol. As mentioned hereinbefore, the cutoff frequency of the diode selected for use with a' circuit of the type described, may be sufficiently low so that spurious or parasitic oscillation is not produced at third harmonics of fol.

A circuit including a current controlled negative resistance diode may include a coaxial resonant transmission line 89 as shown in FIGURE 14. The coaxial transmission line has an inner conductor 90, and an outer conductor 92 which may be of cylindrical configuration. The inner conductor 90, has conductor sections a and 90b of lengths l1 and l2 respectively, which correspond to the lengths of the conductor sections l1 and l2 shown in FIGURE 13.

The resistor 34' is inserted in the center conductor 90 between the sections 90a and 90b, and the current controlled negative resistance diode 94 is inserted in series with the center conductor 90 at the remote end of the section 90b. A suitable coupling device such as a coaxial line 96 of the proper impedance to match the characteristic irnpedance of the quarter-wave section l1, is provided to couple the resonant transmission line 90 to signal source and load circuits represented as the resistances Rs and R1 respectively. It will be noted that for the current con- 9 trolled negative resistance diode, the effective resistances of the source and load circuits are connected in series with the diode.

To prevent phase shifts of the signal in the resonant transmission line 90 which cause reected signals of a phase to interfere with efiicient amplification of the applied signal, the residual reactances in the circuit must be tuned out. These reactances are primarily due to the effective series inductance of the diode, and the shunt capacitance introduced primarily by the line section l2. A tuning stub 98 comprising a coaxial line section having its center conductor connected to the center conductor 9i) adjacent the position of the diode 94, is provided to tune out the residual reactances. If desired the tuning stub may include a capacitive shorting member 100 to permit convenient changing of the effective length of the tuning stub 98.

Since the tuning stub is capactively shorted, the inner and outer conductors 98a and 9811 thereof may be used as the circuit path for the biasing current for the diode 94. The series resistance represented by the resistor 101 of the biasing current source (not shown) must be larger than the absolute value of the negative resistance of the diode 94 for stable biasing in the negative resistance region of the diode characteristic. To prevent the source and load circuits from shunting the biasing supply source, the coaxial line 96 may be capacitively coupled to the resonant transmission line structure.

For amplification, the resistances of the signal source and load circuits are selected to exceed the absolute valve of the diode negative resistance by a slight amount, for signal frequencies within the amplifier frequency bandpass. At frequencies outside of the amplifier bandpass a net positive resistance is presented to the source and load circuits so that parasitic oscillations will not occur.

If the impedance of the circuits connected across the terminals 36' is greater than the characteristic impedance Z0, of the line section l1, then the input resistance characteristic is represented in FIGURE 10. In the case of the current controlled device, as mentioned above, the ordinate values should be read in terms of resistance. Here, as in the case of the structure of FIGURE 8, the envelope drops to a minimum at a frequency fm, where the line section l2 appears as a quarter wave line due to the mismatch of the transmission line with the circuits connected therewith. The dips occurring at the frequency fm and third harmonics thereof can be eliminated by removing the section of the transmission line corresponding to the length l1, as is shown in FIGURES l5 and 16.

The input resistance characteristic of the resonant structure of FIGURES 15 and 16 is shown in the graph of FIGURE 9. The center frequency of the amplifier passband for the structure of FIGURE 16 is the frequency of resonance of the shorted quarterwave line section of length I2. The amplifier of FIGURE 16 includes a current controlled negative resistance diode 102, and a non-capacitive resistor 104. The absolute value of the diode negative resistance is selected to be less than the resistance of the resistor 104. Thus, the amplifier will be stable at frequencies outside of the amplifier passband, and not subject to parasitic or spurious oscillation. The residual reactance in the circuit primarily due to the diode may be tuned out by an adjustable tuning stub 106.

What is claimed is:

1. In an electrical circuit the combination comprising, a transmission line section having a pair of parallel conductors, resistive means connected between the conductors of said transmission line section at one end thereof, a negative resistance device connected between the conductors of said transmission line section at the other end thereof, and means for tuning said circuit connected to the conductors of said transmission line section adjacent the connection of said device.

2. An electrical circuit comprising in combination a 16 negative resistance diode, biasing circuit means for said diode including a stabilizing resistor across which a biasing voltage is developed, and a quarter-wave transmission line impedance transformer coupled between said diode and said resistor to apply said biasing Voltage to said diode, said impedance transformer and said resistor exhibiting to said diode a conductance-frequency characteristic providing a resultant positive conductance that is less than the absolute value of the negative conductance of said diode for a band of frequencies in the operating frequency range of said circuit and a resultant positive conductance which is greater than the absolute value of the negative conductance of said diode for frequencies outside said band of frequencies.

3. An electrical circuit comprising in combination a transmission line having an electrical length substantially equal to one quarter of a wave length at the operating frequency of said circuit, a negative conductance diode coupled to one end of said transmission line, and a resistor coupled to the other end of said transmission line, the resistance of said resistor being small with respect to the characteristic impedance of said transmission line so that said resistor and said transmission line exhibit a smaller positive conductance than the absolute value of the negative conductance of said diode for a band of frequencies in the operating frequency range of said circuit and a greater positive conductance than the absolute Value of the negative conductance of said diode for frequencies outside of said band of frequencies.

4. An electrical circuit comprising in combination a transmission line having fan electrical length substantially equal to one quanter lof a Wavelength at the operating frequency of said circuit, la negative resistance diode adapted -to exhibit a given absolute value of minimum negative resistance coupled to one end of said transmission line and a resistor having .a resistance magnitude less than said absolute value ofthe negative resistance of said diode coupled to the other end of said line, said transmission line having a characteristic impedance greater than the resistance magnitude of said resistor.

5. A high frequency electrical circuit comprising in combination a transmission line open at one end, a resistor coupled to the other end of said transmission line, and a negative resistance diode coupled to said transmission line at a distance from said resistor that is substantially one quarter of a wavelength at the operating frequency of said electrical circuit.

6. A high frequency electrical circuit comprising in combination a transmission line having a resonant first section and Ia second section coupled together at a junction, a resistor coupled to the end of said second section, and a negative resistance diode coupled to said transmitssion line at the junction of said first and second sections, said second section having an electrical length that is substantially one quarter of a Wavelength at the operating frequency of said electrical circuit.

7. An amplifier tunable over 'a broadband of high frequencies comprising in combination a transmission line having a first section and a second section joined together at a junction, said transmission line having a given charactenistic conductance, a resistor coupled to the end of rsaid first section and adapted to be coupled to a vbiasing source, a negative resistance diode adapted to exhibit a given absolute value of maximum negative conductance coupled to said transmission line at the junction of said rst and second sections to comprise the active element of said amplifier, said resistor having a conductance magnitude greater than the maximum negative conductance of said diode for biasing said diode to exhibit said negative conductance, said first section of lsaid transmission line having an electrical length vsubstantially equal .to one quarter of ia wave-length at said predetermined .frequency in said broad band of high frequencies, the combination of said resistor, said diode, and said second Isection of said transmission line having :a conductance-frequency characteristic which exhibits to said first section of said transmission l-ine a resultant negative conductance within said broadband of high fnequencies and a resultant positive conductance outside of said broadband of high frequencies, said first section of said transmission line providing a tunable resonant circuit tor said ampiifier, and means coupling a signal source and a load circuit to said transmission line.

8. An amplifier tunable over a broadband of high frequencies comprising in combination a first transmission line and a second transmission line joined together at a junction, said first and second transmission lines having different characteristic conductances, a resistor coup-led tothe end of said second transmission line and adapted to be coupled to a biasing source, a negative resistance diode 'adapted to exhibit Aa given absolute value of maximum negative conductance coupled to said tnansmission lines at the junction of said first and second tnansmfission lines to comprise the active element of said amplifier, said resistor having a conductance magnitude greater than the maximum negative conductance of said diode for biasing said diode to exhibit said negative conductance, said second transmission line having an electrical length substantially equal to one quarter of a wavelength in said broad band of high frequencies, the combination of said resistor, said diode, and said second transmission line having a conductance-frequency characteristic which exhibits to said first transmission line a resultant negative conductance within said broadband `of high frequencies and la resultant positive conductance outside of -said broadband of high frequencies, said first transmission line having an electrical length which provides a tuned resonant circuit for said amplifier, means for changing the frequency of resonance of said amplifier to tune said amplfiier throughout said broadband of frequencies, a source of signals to be amplified, said signal source having a given positive conductance, a load circuit having a given positive conductanoe, and means coupling said signal source and said load circuit to said transmission line, the sum of said signal source and load circuit positive conductances being greater than the resultant negative conductance of said amplifier throughout said broadband of high frequencies. 9. An electrical circuit comprising in combination, a curnent-controlled negative resistance diode, biasing circuit means Bor :said diode including a stabilizing resistor, and a quarter-Wave transmission line impedance transformer coupled between said `diode and said resistor, said impedance transformer and said resistor exhibiting to said diode a resistance-frequency characteristic providing a resultant positive resistance that is less than the absolute value of the negative resistance of 'sa-id diode for a band of frequencies in the operan-ing frequency range of said circuit and a resultant positive resistance which is greater than the absolute value of the negative resistance of said diode Ifor Afrequencies outside said band of frequencies.

References Cited in the file of this patent UNITED STATES PATENTS 2,274,347 Rust Feb. 24, 1942 2,469,569 Ohi May 10, 1949 2,522,402 Robertson Sept. 1'2, 1950 2,777,9fi6 Shocldey Jan. 15, 1957 2,806,138 Hopper Sept. 10, 1957 OTHER REFERENCES Sommers: Tunnel Diodes `as High-Frequency Devices, Proc. IRE, Iuiy 1959, pages 1201-1206.

Chang: Low-Noise Tunnel-Diode Amplifier, Proct IRE, July 1959, pages 1268, 1269. 

1. IN AN ELECTRICAL CIRCUIT THE COMBINATION COMPRISING, A TRANSMISSION LINE SECTION HAVING A PAIR OF PARALLEL CONDUCTORS, RESISTIVE MEANS CONNECTED BETWEEN THE CONDUCTORS OF SAID TRANSMISSION LINE SECTION AT ONE END THEREOF, A NEGATIVE RESISTANCE DEVICE CONNECTED BETWEEN THE CONDUCTORS OF SAID TRANSMISSION LINE SECTION AT THE OTHER END THEREOF, AND MEANS FOR TUNING SAID CIRCUIT CONNECTED TO THE CONDUCTORS OF SAID TRANSMISSION LINE SECTION ADJACENT THE CONNECTION OF SAID DEVICE. 